A service launched on Tuesday lets cell phone users unmask the Caller ID on anonymous calls.

TrapCall, which is offered by TelTech Systems, reveals the phone numbers of and, in some cases, the names and addresses of blocked Caller IDs. Toms River, N.J.-based TelTech is also the company behind the controversial SpoofCard, an Internet calling-card service that allows users to place calls in which originating caller numbers appear to be something completely different--like the White House switchboard.

Cell phone users have long been able to shield their originating number from display by dialing *67 before placing a call. However, cell calls placed to 800-numbers have been immune to this technique because the toll-free number is paying to receive the call. TrapCall takes advantage of that arrangement.

TrapCall instructs new users to reprogram their cell phones to send all rejected, missed, and unanswered calls to TrapCall's own number. When a blocked or restricted number appears on a cell display, the user presses a button on the cell that is normally used to send the call to voicemail. The call is then rerouted to TrapCall's toll-free line, where the caller's information is obtained and then sent back to the original call's recipient. All this reportedly takes about six seconds while the caller is listening to a normal dial tone.

The service, which is currently available to AT&T and T-Mobile subscribers, is free and includes the option of blacklisting unwelcome callers. Additional premium features include the recording incoming calls (which TelTech notes may be illegal, depending on the users' individual state laws), voicemail transcriptions via e-mail and text message, and the ability to listen to voicemail via the Web.

However, the service raises a couple of sticky privacy issues, especially for victims of domestic violence.

Cindy Southworth, director of technology at the National Network to End Domestic Violence, told Wired that the service raises "huge concerns" for her that abusers would use the service to locate victims fleeing abusive relationships, especially ones in which the victim and abuser share custody of a child.

However, TelTech President Meir Cohen responded to those charges by defending the service as merely utilizing a process that has long been available to anyone with access to an 800-number.